The present invention relates to a method of controlling power to a continuous ink jet print head to maintain proper directionality of a stream of droplets at the end of a printing operation. In particular, the present invention relates to a method of timing a deflection correcting electrical pulse relative to operational pulses of an asymmetric thermal droplet deflector of a continuous ink jet printer.
Ink jet printing has become recognized as a prominent contender in the digitally controlled, electronic printing arena because of various advantages such as its non-impact, low noise characteristics and system simplicity. For these reasons, ink jet printers have achieved commercial success for home and office use and other areas.
Traditionally, color ink jet printing is accomplished by one of two technologies, referred to as drop-on-demand and continuous stream printing. Both technologies require independent ink supplies for each of the colors of ink provided. Ink is fed through channels formed in the print head. Each channel includes a nozzle from which droplets of ink are selectively extruded and deposited upon a medium. Each technology requires separate ink delivery systems for each ink color used in printing. Ordinarily, the three primary subtractive colors, i.e. cyan, yellow and magenta, are used because these colors can produce up to several million perceived color combinations.
In drop-on-demand ink jet printing, ink droplets are generated for impact upon a print medium using a pressurization actuator (thermal, piezoelectric, etc.). Selective activation of the actuator causes the formation and ejection of an ink droplet that crosses the space between the print head and the print medium and strikes the print medium. The formation of printed images is achieved by controlling the individual formation of ink droplets as the medium is moved relative to the print head.
In continuous stream or continuous ink jet printing, a pressurized ink source is used for producing a continuous stream of ink droplets. Conventional continuous ink jet printers utilize electrostatic charging devices that are placed close to the point where a filament of working fluid breaks into individual ink droplets. The ink droplets are electrically charged and then directed to an appropriate location by deflection electrodes having a large potential difference. When no print is desired, the ink droplets are deflected into an ink capturing mechanism (catcher, interceptor, gutter, etc.) and either recycled or discarded. When printing is desired, the ink droplets are not deflected and allowed to strike a print media. Alternatively, deflected ink droplets may be allowed to strike the print media, while non-deflected ink droplets are collected in the ink capturing mechanism. While such continuous ink jet printing devices are faster than drop on demand devices and produce higher quality printed images and graphics, the electrostatic deflection mechanism they employ is expensive to manufacture and relatively fragile during operation.
Recently, a novel continuous ink jet printer system has been developed which renders the above-described electrostatic charging devices unnecessary and provides improved control of droplet formation. The system is disclosed in the commonly assigned U.S. Pat. No. 6,079,821 in which periodic application of weak heat pulses to the ink stream by a heater causes the ink stream to break up into a plurality of droplets synchronous with the applied heat pulses and at a position spaced from the nozzle. The droplets are deflected by heat pulses from a heater in a nozzle bore. This is referred to as asymmetrical application of heat pulses. The heat pulses deflect ink drops between a xe2x80x9cprintxe2x80x9d direction (onto a recording medium), and a xe2x80x9cnon-printxe2x80x9d direction (back into a xe2x80x9ccatcherxe2x80x9d).
While such continuous ink jet printers utilizing asymmetrical application of heat have demonstrated many proven advantages over conventional ink jet printers utilizing electrostatic charging tunnels, it has been noted that at the end of a printing operation, the next droplet or droplets directed toward the gutter may be directed toward the printing medium instead. U.S. Pat. No. 6,254,225 assigned to the assignees of the present application and which is incorporated herein by reference, discloses a method for controlling a terminal flow of ink droplets from the nozzle of an ink jet printer at the end of a printing operation to correct this deficiency. It is noted that because the ""225 patent was not issued until Jul. 3, 2001, it is not prior art with respect to the inventions claimed in the present application.
The cause of such droplet misdirection is not entirely understood but it is believed that this deficiency is caused by the non-instantaneous thermal response time of the heated portion of the nozzle to cool back to ambient temperature. Since the amount of the drop deflection is directly related to the temperature of the ink, and since the heated half of the ink jet nozzle does not cool instantaneously, it is believed that, after the end of a printing operation, the first ink droplet formed is misdirected away from the ink gutter and toward the printing medium due to the residual heat of the ink jet nozzle. Whether or not the second or third subsequent droplets are similarly misdirected is dependent upon the residual heat of the print head in the vicinity of the nozzles, the viscosity and thermal properties of the ink, and other thermal and fluid dynamic factors. Any such misdirected droplets can interfere with the objective of obtaining high image quality printing from such devices.
To correct the above described deficiency, the ""225 discloses a printer having a first heater element disposed on one side of the nozzle that is selectively actuated to direct ink droplets away from a recording medium and into an ink gutter during a printing operation. The printer also has a second heater element disposed on the side of the nozzle opposite from the first heater element. After the first heater element applies its last operational heat pulse to the printing nozzle at the end of a printing operation, the second heater element applies at least one deflection correcting heat pulse of the same duration, magnitude and period as the last operational heat pulse. The method as described in the ""225 reference prevents ink droplets generated after the end of a printing operation from erroneously striking the printing medium.
Whereas a method for preventing ink droplets generated after the end of a printing operation from erroneously striking the printing medium is provided in the ""225 reference, an accurate and efficient method for controlling the deflection correcting electrical pulse provided to the second heater element disposed on the side of the nozzle opposite from the first heater element is not disclosed.
In the above regard, the present inventors recognized that efficient and accurate timing of the electrical pulse that operates the second heater element is not known. Moreover, it has also been recognized that in certain applications, it may be desirable to adjust the timing of the electrical pulse that operates the second heater element.
In view of the above, one advantage of the present invention is in providing an accurate and efficient method for preventing misdirection of ink droplets at the end of a printing operation.
In this regard, another advantage of the present invention is in providing a method for controlling the timing of the deflection correcting electrical pulse for the second heater element disposed on the side of the nozzle opposite from the first heater element.
In accordance with the preferred embodiment of the present invention, these advantages are obtained by a method for timing a deflection correcting electrical pulse relative to operational pulses of an asymmetric thermal droplet deflector of a continuous ink jet printer having plurality of nozzles, comprising the steps of generating at least one line image data with a plurality of image data values corresponding to the plurality of nozzles, the plurality of image data values being indicative of desired pixel graytone levels for the plurality of nozzles, comparing the plurality of image data values to a reference value, generating at least one serial bit stream in the form of serially arranged bits based on the comparison of the image data values to the reference value, and producing an actuation value that times the deflection correcting electrical pulse in the serial bit stream when the image data value is equal to the reference value.
In accordance with one embodiment, the method also includes the step of generating the deflection correcting electrical pulse timed to the actuation value in the serial bit stream. In still another embodiment, the method also includes the step of iteratively comparing the plurality of image data values of the line image data with the reference value.
The actuation value produced to time the deflection correcting electrical pulse is a digital 1. In one embodiment, the reference value increases in uniform increments. In this regard, the method may further include the step of generating the deflection correcting electrical pulse timed to the actuation value in the serial bit stream. The method may also include the step of iteratively comparing the plurality of image data values of the line image data with the reference value as the reference value is increased in uniform increments. The reference value may be started at 1 so that the deflection correcting electrical pulse is generated concurrently timed with last operational pulses for each of the plurality of nozzles. Alternatively, the reference value may be started less than 1 so that the deflection correcting electrical pulse is generated subsequent to last operational pulses for each of the plurality of nozzles. For instance, the reference value may be started at 0, xe2x88x921, or xe2x88x922 so that the deflection correcting electrical pulse is generated one, two, or three predetermined time periods respectively, subsequent to last operational pulses for each of the plurality of nozzles.
In accordance with one embodiment, the total number of iterations of comparing the plurality of image data values of the line image data with the reference value is less than or equal to the total number of pixel graytone levels. In another embodiment, the total number of iterations exceeds the total number of pixel graytone levels.
In accordance with yet another embodiment of the present invention, the reference value is a plurality of reference values stored in a look up table. In this regard, at least first of the plurality of reference values is 0 so that the deflection correcting electrical pulse is generated subsequent to last operational pulses for each of the plurality of nozzles. In addition, the method may further include the step of iteratively comparing the plurality of image data values of the line image data with the plurality of reference values stored in the look up table.
In accordance with still another aspect of the present invention, a method for timing a deflection correcting electrical pulse relative to an operational pulse of an asymmetric thermal droplet deflector of a continuous ink jet printer is provided, the method comprising the steps of generating line image data with plurality of image data values indicative of desired pixel graytone levels, iteratively comparing the plurality of image data values to a reference value at predetermined time periods, generating a serial bit stream with an actuation value based on the comparison of the plurality of image data values to the reference value, and generating the deflection correcting electrical pulse based on the actuation value, the deflection correcting electrical pulse being timed within a predetermined number of time periods of the operational pulse.
In accordance with one embodiment, the deflection correcting electrical pulse is generated in the same time period of the operational pulse. Alternatively, the deflection correcting electrical pulse is generated in a time period subsequent to the operational pulse. In this regard, the deflection correcting electrical pulse is generated in one or two time periods subsequent to the operational pulse. In one embodiment, the reference value increases in uniform increments, while alternatively, in another embodiment, the reference value is a plurality of reference values stored in a look up table.
These and other advantages and features of the present invention will become more apparent from the following detailed description of the invention when viewed in conjunctions with the accompanying drawings.